The aim of this investigation is to assess in human subjects whether measurement of brain electrical activity is an adequate clinical monitor of cerebral oxygenation during anesthesia and surgery. Both induced hypotension and hemodilution are being used as anesthetic adjuncts to reduce blood loss during major surgery and both may reduce cerebral oxygenation. We will monitor the spontaneous electroencephalogram during surgery in patients undergoing controlled hypotension with or without hemodilution. Extensive preoperative and postoperative neuropsychological testing will be performed utilizing the Halstead-Reitan battery and other sensitive indicators of organic brain injury in an attempt to detect minimal hypoxic injury. Our goal is to determine the significance of changes in electrical activity as a predictor of hypoxic brain injury. These clinical studies will complement a research project in animals currently in progress, in which neurological damage is induced by hypotensive hypoxia and correlations with spontaneous electroencephalographic activity and evoked potentials are being sought. Both of these studies should clarify the correlation between electrical activity and neuron viability as well as determine the adequacy of brain electrical activity as a reliable monitor of brain hypoxia.